Researchers observed bright flares of X-rays that are generated when gas falls into a supermassive black hole. The flares echoed from the gas falling into the black hole, and as the flares subsided, brief X-ray flashes were seen – corresponding to the reflection of the flares from the other side of the disk, bending around the black hole’s strong gravitational field. (Image credit: Dan Wilkins)
The researchers fulfill a prediction of Einstein’s general theory of relativity and report the very first record of X-ray emissions from the other side of a black hole.
When observing the X-rays coming from the supermassive black hole at the center of an 800 million light years distant galaxy were hurled into the universe, Stanford University astrophysicist Dan Wilkins noticed a fascinating pattern. He observed a series of bright X-rays – exciting but not unprecedented – and then the telescopes picked up something unexpected: additional flashes of X-rays that were smaller, later and in different “colors” than the bright flares.
According to theory, it was true those glowing echoes coincide with X-rays reflected from behind the black hole – but even a basic understanding of black holes tells us that this is a strange place for light.
“Any light that enters this black hole, doesn’t come out, so we shouldn’t be able to see what’s behind the black hole, ”says Wilkins, a researcher at the Kavli Institute for Particle Astrophysics and Cosmology at Stanford and SLAC National Accelerator Laboratory. However, there is another strange property of the black hole that makes this observation possible.
“The reason we can see this is that this black hole distorts space, bends light and rotates magnetic fields around itself”, explains Wilkins.
The strange discovery, detailed in an article in Nature, is the first direct observation of light behind a black hole – a scenario that predicted Einstein’s general theory of relativity but has never been confirmed / p> “When astrophysicists began speculating fifty years ago about how the magnetic field might behave near a black hole, they had no idea that one day we might have the techniques to observe this directly and Einstein’s general Seeing relativity in action, ”says Roger Blandford, co-author, Stanford Professor of Physics and SLAC Professor of Particle Physics and Astrophysics.
Tue The original motivation behind this research was to learn more about a mysterious feature of certain black holes called a corona. Material falling into a supermassive black hole powers the brightest continuous light sources in the universe, forming a corona around the black hole. This light – X-ray light – can be analyzed to map and characterize a black hole.
The leading theory of what a corona is begins with gas slipping into the black hole, where it can accumulate in the millions Degree overheated. At this temperature, electrons separate from atoms, creating a magnetized plasma. Captured by the strong rotation of the black hole, the magnetic fields bent so high above the black hole and swirled around themselves so much that it eventually breaks completely – a situation so reminiscent of what is happening around our own sun call it the “corona”.
“This magnetic field, which is bound and then snaps close to the black hole, heats everything around it and creates these high-energy electrons that then create the X-rays,” says Wilkins.
When Wilkins looked more closely to examine the origin of the torches, he saw a series of smaller flashes. The researchers found that these were the same X-rays, but reflected off the back of the disk – a first look at the other side of a black hole.
“For a number of years I’ve been making theoretical predictions about how these echoes appear to us, ”says Wilkins. “I had already seen it in the theory I was developing. So when I saw her doing the telescope observations, I was able to find the connection. ”
The mission to characterize and understand Coronas continues and requires more observation. Part of this future will be the European Space Agency’s X-ray observatory, Athena (Advanced Telescope for High-Energy Astrophysics). As a member of the laboratory of Steve Allen, professor of physics at Stanford and of particle physics and astrophysics at SLAC, Wilkins is helping develop part of the Wide Field Imager detector for Athena.
“It has a much larger mirror than we’ve ever had it on an X-ray telescope, and it will allow us to get higher resolution images in much shorter observation times, ”says Wilkins. “The picture that we are currently gaining from the data will be much clearer with these new observatories.”
Further co-authors come from Saint Mary’s University (Canada), the Netherlands Institute for Space Research (SRON), and the University of Amsterdam and Penn State.
This work was supported by NASA’s NuSTAR and XMM-Newton Guest Observer programs, a Kavli grant from Stanford University, and the VM Willaman Foundation in Penn State.
DOI: 10.1038 / s41586-021-03667-0